BIOAUGMENTATION CONSIDERATIONS - Bioaugmentation for Groundwater Remediation - page 166

materials and they can be readily transported in the aqueous phase (Schaefer et al., 2009 ), which
is consistent with early field observations (Ellis et al., 2000 ; Lendvay et al., 2003 ; Major et al.,
2002 ). Further, the Dhc subpopulations that move through the aquifer may be compositionally
different than the original inoculum (Behrens et al., 2008 ), and they may have a greater specific
activity that the original inoculum (Schaefer et al., 2009 ).
Correlations Between Inoculum Density and In Situ
Dechlorination Rates
Despite the extensive laboratory research performed to evaluate bioaugmentation with
Dhc -containing consortia, and the application of the technology at several hundred sites,
no clear correlations between inoculum density and in situ dechlorination rates have emerged.
The only study focused on correlating in situ Dhc levels in the field to degradation activities are
those discussed above by Lu et al. ( 2006 ) and that study focused on natural Dhc populations
rather than those added as a bioaugmentation remedy.
Only a few field-scale bioaugmentation projects have been described in detail, and many
of these were pilot studies performed early in the development of Dhc -containing cultures
for VOC remediation before quantitative polymerase chain reaction (qPCR) techniques
were available to quantify the numbers of Dhc present in the aquifers. In a recent study,
Lee et al. ( 2008 ) measured Dhc numbers and reductive dehalogenase gene transcripts to
evaluate Dhc performance and activity at a bioaugmentation site. The site, located at Fort
Lewis, WA, USA, contained TCE at DNAPL concentrations, and bioaugmentation was per-
formed in two plots containing a recirculation system. During the 3 months after culture
injection, VC reductase gene ( vcrA ) copy numbers increased by as much as 2 orders of
magnitude in the plots, but no attempts were made to correlate Dhc or reductive dehalogenase
numbers to in situ dechlorination rates. In a similar recent study (Scheutz et al., 2008 ),
successfully bioaugmented a site with Dhc and found that the Dhc numbers increased to
approximately 10 8 cells/L after 76 days, and remained relatively constant throughout most of
the remainder of the demonstration.
Most recently, Schaefer et al. ( 2010a ) evaluated the effect of inoculation density on
dechlorination rates in a silty sand aquifer at Fort Dix, NJ, USA (Steffan et al., 2010 ). The
aquifer at the site had native Dhc , but the indigenous strains were unable to dechlorinate DCE
or VC, resulting in a DCE stall at the site. Furthermore, the aquifer had a low natural pH (pH
< 5) that required pH adjustment in order to achieve complete dechlorination, even with an
added inoculum. The study used four recirculation loops initially designed to achieve a 30-day
travel time between the injection and extraction wells. Three loops were inoculated with 10X, 1X,
or 0.1 X of the predicted necessary inoculum. The predicted inoculum density was based on an
even distribution of the added cells within the treatment zone to an aqueous Dhc concentration
of 10 7 Dhc /L. Figure 5A.1 shows the VOC concentrations in the second row of monitoring wells
(20 ft [6 m] from the injection well) in the three inoculated recirculation loops. Most notably,
ethene was produced in all three inoculated recirculation loops, including loop three (BMW 6)
which received the lowest inoculum volume (i.e., 100-fold less than loop 1 [BMW2]). No ethene
was produced in loop 4 which was not inoculated but did contain native Dhc strains. The study
concluded that many factors affect the amount of culture needed for effective treatment and
that selecting the amount of culture needed cannot reliably be based solely on the amount of
groundwater to be treated. A 1-dimensional model has been developed to aid practitioners in
determining the amount of culture needed (Schaefer et al., 2009 ), and the utility of the model for